1. Field of the Invention
This invention relates generally to frequency counters having electronic circuits especially dedicated to measurement of the frequency of external alternating voltages, and more particularly, to battery operated frequency counters having electronic circuits which can count input frequencies of at least one thousand one hundred megahertz (1.1 Ghz).
2. Description of the Prior Art
Before digital frequency counters became relatively inexpensive and practical, high frequency measurement usually involved comparison of an accurately known frequency with an unknown frequency through usage of an interpolation oscillator or other transfer means. A popular prior and present method of frequency measurement used or uses a high frequency multivibrator which was or is locked on to a highly stable and accurately calibrated crystal controlled oscillator having an oscillation frequency of from 50 to 100 kilohertz. The output of the multivibrator containes a multiplicity of frequencies harmonically related to the source oscillator which can be used for comparison with an unknown frequency subject to correct identification of the harmonic frequency of interest.
A classical prior measurement method which does not require comparison with a reference for use at very high frequencies (VHFs) and ultra high frequencies (UHFs) consists of measurement of current or voltage minima on energized open wire transmission lines, so-called lecher wires, slotted coaxial lines or plug adjusted cavities or wave guide sections. These devices are actually wave meters, but the measurements are related to frequency as follows: ##EQU1## Where the propagation velocity on low-loss lines or in cavities was known, a measurement accuracy of about 2% was possible at UHFs.
Frequency meters for use at power line frequencies (50-60 Hz) usually consisted of crossed coil discriminators with one coil resonant above the frequency of interest and one coil resonant below the center frequency driving a soft iron indicator element.
Vibrating reed-type frequency indicating instruments are also known and widely used. In a resonant reed instrument, the reeds are mounted horizontally behind a viewing window. The reeds are driven by a common coil and, if the reeds are polarized or magnetically biased, they are adjusted to resonate at selected intervals above and below a center frequency of interest. If no polarizing magnet is used, the reeds are configured to vibrate at twice excitation frequency. Popular center frequencies for reed frequency meters are 25, 50, 60 and 400 hertz. Reed frequency meters are widely used on portable generating equipment and on special purpose frequency changers. Accuracies of from 1 hertz to 0.1 hertz are common.
A Wien bridge provides another convenient method of frequency measurement over the audio range up to 30 or 40 kilohertz. Use of a low-pass filter in the null detector circuit of the Wien bridge avoids the masking effect of harmonics of the fundamental frequency to provide an accuracy of one percent.
Prior art frequency meter devices or techniques should not be used to count frequencies, but only to provide a comparison with a known frequency. Prior frequency counting techniques involved the accumulation of an unknown number of cycles of an alternating voltage or pulses of a square voltage wave form within an accurately denominated time period such as one second and then digitally display the resulting total.
One class of prior frequency counters used electromechanical techniques comprising a polarized reed relay and a high speed mechanical accumulator gated by a mechanical stop clock with a counting capability of up to 500 hertz. These devices were used as interpolators for comparing an unknown with a known frequency, as tachometers and as events counters.
Modern digital frequency counters based on the use of integrated circuit devices or technology are flexible instruments with measurement capability from a few hertz to megahertz frequencies. However, These prior digital frequency counters could not be made portable (battery operated) and count frequencies in excess of about 100 megahertz.
Renewed interest in UHF television and new FCC allocations of communication and amateur bands above 800 Mhz has demonstrated a need for accurate frequency counting or measurements up to 1000 Mhz and beyond. Accordingly, a need existed for a portable (battery operated) highly accurate, digital frequency counter which could count frequencies to at least 1.1 Ghz.